The year 2016 will be one second longer than usual, thanks to a “leap second” tacked on just at year’s end.

When the clock strikes midnight at Greenwich Mean Time on New Year’s Eve, official clocks around the world will pause for exactly one second before changing over to the New Year.

For anyone on eastern time, the changeover will be visible just before 7 p.m. on December 31. Web clocks (and the clocks displayed on 24-hour news channels) will hold at “6:59:60” before clicking to “7:00:00.”

(Super nerds who are keen on spending New Year’s Eve witnessing the leap second can go here, to the official web clock of the National Research Council of Canada.)

(Photo credit should read HO/AFP/Getty Images)

The leap second is a product of the atomic age. In an era of super-accurate atomic clocks, scientists discovered that the daily rotation of the earth isn’t as predictable as previously thought.

While the average earth day lasts 86,400 seconds on paper, in practice NASA estimates it can be as long as 86,400.002 seconds.

The reason for this is that the earth is slowing down.

Between shifting ice packs, earthquakes, storms and the gravitational tug of war between the moon and sun, it all conspires to imperceptibly slow down the twirling of the earth.

Dave Abel / Toronto Sun / QMI Agency.

In other words, you can no longer set your watch to the sun. Since the year 2000, four extra “leap seconds” have already been tacked onto the world’s clocks.

And unlike leap years, leap seconds are not predictable.

Scientists need to keep close tabs on the rotation of the earth using the world’s network of radio telescopes. Whenever the orbit starts looking wonky, a leap second is scheduled (here, for instance, is a bulletin announcing the Saturday leap second).

Since the practice began in 1972, the longest gap between leap seconds has been seven years. The shortest has been six months. Saturday’s leap second will be the first since June 30, 2015.

It is not possible to alter the Earth’s rotational speed to match the atomic clocks

Leap seconds are not an uncontroversial human practice. Every time one is scheduled, tech companies have to update their code to avoid a miniature Y2K breaking out. But the idea of the leap second, say proponents, is to stop these inaccuracies from adding up.

Almost half a minute’s worth of leap seconds have been added since the 1970s – enough to start screwing with technologies that require razor-sharp precision.

“We can easily change the time of an atomic clock, but it is not possible to alter the Earth’s rotational speed to match the atomic clocks,” said a recent statement by the U.S. Naval Observatory.

The momentous task of rejiggering the world’s clocks falls to a largely unknown body called the International Earth Rotation and Reference Systems Service. Based in Frankfurt, Germany, it’s a body of leading international scientists who meet, exchange emails and otherwise coordinate the herculean task of keeping an eye on the earth’s rotation.

In fact, if you find yourself at the Park Lodge Hotel in Banff this March, you may accidentally share an elevator with one of these modern time lords. The IERRSS is meeting there for a Geodesic Missions Workshop.